1. Field of the Invention
The present invention relates generally to superconductor materials formed from powdered metal oxides and, more particularly, to a method for producing long length high temperature superconductor coils.
2. Description of the Prior Art
Superconductor wires have, for many years, been fabricated according to a conventional technique known as the "powder-in-tube" method. One example of such method is described in U.S. Pat. No. 4,906,609 and illustrated in FIG. 2. Typically, the method involves utilizing initial starting materials and providing for a series of grinding and heat treatment operations to produce a powdered mixture which is then packed into the silver tube. Thereafter, a series of cold working operations including drawing and rolling are performed to thereby provide a desired thickness of the wire. An intermediate heat treatment is introduced followed by rolling to final thickness and thereafter, annealing. The wire is then formed into a magnet coil and the final heat treatment operation is performed on the magnet coil.
A number of problems have been found with the final magnet coil thus produced. One problem relates to the introduction of microcracks. Generally, such cracks are introduced during the rolling operations while producing the final thickness. For example, we have found that the critical current density (Jc) when pressed and heat treated repeatedly will show improvement as a function of time. However, under similar conditions of temperature and time the Jc for repeated rolling/heat treatments does not show improvements. In contrast, the Jc decreases as the rolling steps increase. The graph of FIG. 1 illustrates the differences achieved when pressing as opposed to rolling with respect to total heat treatment time. The plot of the two pressing samples illustrate results obtained when pressing at a pressure of 18 Kbar and at varied pressures. The plot of the two rolling samples illustrate results obtained when rolling operations are performed two and three times. During the rolling operations we find an increased development of microcracks. This is a consequence of the non-uniform application of stresses from the rolls. The enhancement of the critical current density by using uniaxial pressing instead of cold rolling, to produce the final thickness of the wire, prior to annealing is disclosed in Applied Physics Letters, Volume 60, Number 4, Jan. 27, 1992 pages 495-497.
At present in the fabrication of long lengths of monocore HTS tape conductors, critical current density (Jc) properties are substantially lower than what may be obtained in short pressed samples. For example, the best critical current density at liquid nitrogen temperatures (77K) is nearly 70,000 A/cm.sup.2 in Zero field obtained in short pressed BSCCO tapes. However, such high current densities have not been achievable in long length commercial coils. One of the main reasons for the marked differences in Jc properties for short pressed verses longer rolled tapes comes from the fact that the short pressed samples undergo a uniform or uniaxial stress whereas for rolling the stress is non-uniaxial. Consequently, non-uniform stresses may introduce microcracks which may not be completely annealed out. Since density and texture are important to promoting optimum electrical properties, fabrication by rolling of long lengths of HTS tape may severely limit the Jc properties available and therefore eliminate applications with high field requirements at higher temperatures. Properties obtained by pressing or rolling very short lengths of wire can be seen from the Japanese Journal of Applied Physics, Volume 30, Number 12B, December, 1991, pages L2083-L2084. In the method used in this paper, coils of very short lengths were used to obtain the Jc's reported. Using greater than very short lengths would not be practical for pressing, in that, variations in thickness or severe discontinuities would be induced. To fabricate long lengths of HTS tapes with the high Jc's obtained through the method of pressing has heretofore, not been possible.
It has also been suggested to deform Bi-2223 tapes by hot rolling. Such was introduced by R. Flukiger, A. Perin and E. Walker from the Universite de Geneve in Switzerland at the Materials issues in High Temperature Superconductivity Spring Symposium T on Apr. 12-16, 1993. A deformation technique was developed to enhance the degree of texturing as well as the density by hot rolling. A prototype rolling machine was constructed with rolls of 80 mm diameter that can be heated up to 800.degree. C. Various problems were encountered, each requiring separate solutions. The most important are: a) the flow of the Ag sheath at elevated temperatures leads to enhanced sausaging, b) sticking of the Ag sheath to the heated rolls, and c) the precise determination of the tape temperature between the two rolls.
While there is continued progress at providing increased critical current densities for high Tc superconductor materials, further improvements are still needed. Furthermore, when providing long length coils of such wire, the severe discontinuities and microcracks introduced into the high Tc materials using the currently known methods continue to detract from the improved current densities needed in such materials.
The present invention introduces a method of coil fabrication which departs from the normally accepted methods of producing long length superconductor wires, avoids the problems encountered in producing long length superconductors and exploits the ideal conditions obtained by known methods.